1. Field of the Invention
The present invention relates to a photoelectric conversion device including single crystal silicon or polycrystalline silicon and to a manufacturing method thereof.
2. Description of the Related Art
Since the situation of global warming has been serious, a measure against the global warming is an issue that needs to be addressed worldwide. Most greenhouse effect gases which are regarded as a main cause for global-warming, such as carbon dioxide, are exhausted by consumption of energy, such as oil, coals, and natural gases. However, such energy is indispensable to the industrial society. Therefore, the amount of energy consumption cannot be reduced simply. Thus photoelectric power generation which discharges less amount of carbon dioxide and which is eco-friendly has attracted attentions as the next-generation energy.
For the photoelectric power generation, a silicon photoelectric conversion device, which converts light energy into electric energy using semiconductor characteristics, is widely applied. Since silicon photoelectric conversion devices typified by solar cells are already available in the market and governments around the world support solar cells, the production thereof has been expanding year by year. For example, the production of energy generated by solar cells around the world in 2006 is 2521 MW, which has increased by over 40% per annum.
However, there are many obstacles to the widespread of photoelectric power generation. One of the obstacles is a higher cost of photoelectric power generation compared with a cost of commercial power. In order to reduce a cost for photoelectric power generation, improvement of efficiency in a solar cell and reduction of manufacturing costs are indispensable.
Further, the present obstacle to the widespread of photoelectric power generation is short supply of silicon. The supply-demand balance of silicon had been excess in supply reflecting semiconductor recession. In contrast, silicon is now short of supply since around fiscal 2005 because of drastic growth of the solar cell market in addition to the recovery of the semiconductor (LSI) industry. Though major silicon suppliers in the world have already tried to increase capability of silicon production, the increase in demand outweighs the capability.
While shortage of silicon materials is obvious, a thin film silicon photoelectric conversion device has attracted attentions. In the thin film silicon photoelectric conversion device, thin film silicon is formed over a supporting substrate and the thin film silicon functions as a photoelectric conversion layer. In contrast, a balk silicon photoelectric conversion device, which is mainly used currently, whose main portion is formed from a single crystal silicon substrate or a polycrystalline silicon substrate having a thickness which is more than several tens times or more as thick as a thickness necessary for the photoelectric conversion device. Therefore, it is hard to say that silicon is used effectively in the balk silicon photoelectric conversion device. Speaking of extremes, most part of the single crystal silicon substrate or the polycrystalline silicon substrate used for the balk silicon photoelectric conversion device functions only as a structural body for keeping the shape of the photoelectric conversion device. The thin film silicon photoelectric conversion device can have a structure in which silicon is used only for a region for performing photoelectric conversion. Therefore, a silicon usage can be reduced drastically compared with a silicon usage of the balk silicon photoelectric conversion device.
However, the thin film silicon is formed over the supporting substrate in the thin film silicon photoelectric conversion device by using a physical or chemical growing method. Therefore, photoelectric conversion efficiency of the thin film silicon photoelectric conversion device is lower than that of the balk silicon photoelectric conversion device. Thus, a method for manufacturing a solar cell which employs an ion implantation separation method for obtaining a crystal semiconductor layer has provided (for example, see Reference 1: Japanese Patent Application Laid-Open No. H10-335683). A predetermined element is implanted into a crystalline semiconductor so as to be a layer shape and the crystalline semiconductor is bonded over an electrode. A space distributed in a layer shape is formed in the region to which the predetermined element is implanted in the crystalline semiconductor and the crystalline semiconductor is separated along the space by heat distortion to obtain a solar cell A including a crystalline semiconductor layer over the electrode layer. Further, a solar cell B including an amorphous silicon layer is formed thereover to form a tandem solar cell. In this method, a single crystal silicon solar cell which is to be a first power generating layer is formed; however, the thin film silicon photoelectric conversion device which achieves photoelectric conversion efficiency as high as that of the balk silicon photoelectric conversion device is not realized actually.
In order to improve photoelectric conversion efficiency of the photoelectric conversion device, a technique of light confinement in which incident light is used efficiently is researched for all types of the photoelectric conversion device including bulk-type and thin film-type. As the technique of light confinement, formation of a surface texture structure (surface uneven structure) or an antireflection layer on a light incident surface are known. In the case of single crystal silicon, it is known that a surface uneven structure is formed by etching treatment with an alkaline solution or the like using a crystal plane orientation of silicon. Further, in the case of having various plane orientations, such as that of polycrystalline silicon, a method is provided, in which a surface uneven structure is formed using combination of a laser-processing technique and a chemical etching (see Patent Application Laid-Open No. 2003-258285).